Fuel feed for multi-stage carburetors



April 30, 1957 J. F. ARMSTRONG FUEL FEED FOR MULTI-STAGE CARBURETORS Filed March 25, 1954 5 Sheets-Sheet 1 JAMES F. ARMSTRONG l April 30, 1957 J. F. ARMSTRONG 2,790,630

FUEL FEED FOR MULTI-STAGE CARBURETORS Filed March 213, 1954 5 sheets-sheet 2 IN VEN TOR.

JAMES F. ARM ST RONG Haya@- AT TRNEY April 30, 1957 J. F. ARMSTRONG 2,790,630

FUEL FEED FOR MULTI-STAGECARBURETORS Filed March 23. 1954 5 Sheets-Sheet 3 /\'g2 A S JNVENTOR. JAMES F. ARMSTRONG ATTORNEY April 30, 1957 J. F. ARMSTRONG 2,790,530

FUEL FEED FOR MULTI-'STAGE CARBURETORS Filed March 23, 19:54v

5 Sheets-Sheet 4 INVENTOR.

JAMES F. ARMSTRON ATTORNITV April 30, 1957 J. F. ARMSTRONG 2,790,630

FUEL. FEED FOR MULTI-STAGE cARBuREToRs 5 Sheets-Sheet 5 Filed March 23, 1954 INVENTOR. JAMES F. ARMSTRONG BY @7L ATTORNEY nited States Patent 2,790,630 l u FUEL FEED non MULTI-STAGE cARBURnrous James F. Armstrong, St. Louis, Mo., assigner, by rnesne assignments, .to ACF Industries, Incorporated, New York, N. Y.,a corporation of New Jersey Application March 23, 1.954, Serial No. ,418,049 18 Claims. ,(Cl. 261-23) This invention relates to two-stage carburetion and fuel supply devices for internal combustion engines.

The invention incorporates `the following advantageous, lnovel and interrelated features:

1. More vaccurate automatic control of the secondary throttle valve without increasing the height or length of the carburetor, asis necessary where an extra, automatic valve is provided.

2. Provision for insuring firm closing of the secondary throttle valve without increasing the strength of the secondary throttle closing spring and, thereby, the force needed to open this throttle.

3. Means insuring prompt issue of fuel from the secondary main fuel nozzle, upon Vinitiation of opening of the secondary throttle, so as to avoid a flat spot frequent- 'ly noted at this point.

4. Means preventing opening of the secondary throttle valve as long as the choke valve in the air entrance f the primary mixture conduit is in a substantially restricting position, .without the provision of the usual stops and locking devices heretofore used for 4this purpose.

5. Means limiting the opening force applied to the secondary throttle to insure prompt closing of the throttle when the operator releases the accelerator pedal.

The invention is illustrated as embodied in a fourbarrel type of carburetor, but applies to any two-stage mixture supply system in which the second fuel supply means becomes operative after a predetermined, initial opening movement of the primary throttle valve.

In the accompanying drawings:

Fig. l is a side view of a carburetor embodying the invention, portions being broken away and sectioned to illustrate the underlying structure and both throttle valves being shown in their idling positions.

Fig. 2 is a vertical, transverse section through the carburetor illustrating the fuel feeding parts.

Fig. 3 is a side View of the carburetor with certain parts broken away and illustrating the effect of the partially closed choke valve upon secondary throttle operating mechanism.

Fig. 4 is a View similar to Fig. 3 but with the secondary throttle partially opened.

Fig. 5 is a top view of the linkage mechanism interconnecting the primary and secondary throttle valves.

Figs. 6 and 7 show a modified form of the carburetor in the previous figures, with the throttles in different positions.

The carburetor is of the currently popular four-barrel type, but only two of the barrels are shown in the views and the description will proceed on the basis of these two barrels only, it being understood that the primary and secondary instrumentalities may be duplicated, if desired. A pair of adjacent, downdraft mixture conduits are shown, namely, the primary conduit 10 and secondary conduit 11. The conduits have air entrance portions 12 and 13 divided by partition 14 and each has a pair of venturi tubes 15, 16 and 15a, 16a forming the mixing chamber portion of the conduit. At the lower or discharge ends of the mixture conduits are primary throttle valve 17 and secondary throttle valve 18 having operating levers 19 and 20 secured, respectively, thereto and movable therewith. An unbalanced choke valve 21 is pivotally mounted on a shaft 22 in air inlet portion 12 of the primary mixture conduit and is controlled by any suitable mechanism, such 7 7 Patented Apr. 30, l19.57

as the well known automaticchoke control mounted in housing 23 on the outside of airhorn structure 24 which forms the air entrances.

As best shown in Fig. 2, constant level chambers, as at 25 and 2.6, are provided adjacent each mixture conduit and supplied with fuel through a common inlet fitting 27 and past needle valves'28 and 29 and operated by .yokes 30 and 31 carrying the usual oats, as at 31a (Fig. l). Preferably, constant level chambers 25 and 26will be entirely separated so as to minimize the yeffect of surging in the fuel. At the bottoms of the constant level chambers are the metering orifice .elements 32, 33 for metering fuel to primary main fuel passage 34 and secondary main fuel passage 35 Vdischarging into lventuri tubes' 15 and 15a in the primary and secondary conduits, respectively, through nozzles 34a and 35a. Secondary main nozzle 35a discharges slightly below normal fuel level X-X .in bowl 26, while primary main nozzle 34a discharges slightly .above the normal fuel level, as customary. Metering orifice elements 32 and 33 are controlled by metering pins 37 and 38 operated .from the throttle valves by suitable linkage. Metering pin 38 seats upon and closes secondary i IIC@ .metering orifice element 33 when secondary throttle valve 18 is closed. This metering vpin is operated from throttle valve 18 by means of a link 39, crank 40, countershaft 41 rigid thereof, and a small crank 42 rigid with the countershaft and carrying the metering pin through a link 43. Primary metering pin 37 variably restricts metering orice element 32, according to the position of primary throttle 17, as customary, by means or" similar mechanism (not shown). Fuel for idling -is supplied through idle restricting tubes 45 and 46, connecting main fuel passages 34 and 35 with idling passages 47 and 48 leading to ports 49 and 50 adjacent and posterior to the primary and secondary throttle valves, respectively, when closed. A mixture adjusting screw 51 is provided in connection with one of the idling ports 49 on the primary side.

A lever 54 is secured to the end of primary throttle shaft 55 opposite previously mentioned lever 19 (Fig. 2) and is provided with a hole 56 for connection of linkage extending to the accelerator pedal in the drivers compartment for manual control of the primary throttle against the usual throttle return spring (not shown). The normal idling position of the primary throttle is determined by an adjusting screw 57 disposed to engage a stop 58. A suitable fast idling stop of any known construction, preferably, will be provided to increase the engine idling speed at low temperatures when the choke valve is in a restricting position.

Secondary throttle valve 18 is controlled by a suction motor, generally indicated at 60 (Figs. l, 3, and 4). This motor is formed of a pair of housings 61 and 62 which clamp a flexible diaphragm 63 between the flanged, abutting edges thereof. An actuating stem 64 is secured centrally to the diaphragm by means of large washers 65 and 65a and passes outwardly through a guide packing 67 in casing wall 62. The inner, free end of stem 64 slides in a guide 68 located centrally in the end of casing member 61. Stem 64 is pinned at its outer end to a link 66 which is pivotally connected to one end of previously mentioned, secondary throttle actuating lever 20.

Motor housings 61 and 62 form pressure chambers 69 and 70 to which are connected passages 71 and 72 leading, respectively to the throat portion of main venturi tube 16 in the primary mixture conduit and chamber 73 in the air entrance portion of conduit 10 between venturi tubes 1S and 16 and choke Valve 2l. Passage 72 extends through rib 74 which supports venturi' tube 15 and opens upwardly against the direction of air ow into the mixture conduit in the manner of a pitot tube. Passage 71 opens ush through the wall of venturi tube 16 so as to transmit the static pressure condition caused by the velocity of air flow through the venturi tube. Thus, suction motor chambers 70 and 69 and diaphragm 63 are exposed to the differential pressures which exist between the air entrance portion of the primary mixture conduit, posterior to choke valve 21, and the mixing chamber portion of the primary conduit. Diaphragm 63 will be urged rightwardly, as the rate of air ow through venturi tube 16 increases. However, opening of the secondary throttle is prevented by means to be described hereafter, until a predetermined rate of primary air flow is reached and, consequently, the associated engine is operating at a predetermined speed. The location of passage 72 posterior to choke valve 21 provides for substantially reducing the pressure difference applied to motor chambers 69 and 70 as long as the choke valve is in a substantially restricting position, so that no movement of diaphragm 63 and the secondary throttle valve can then occur.

Lever 19 secured to primary throttle shaft 55 extends away from the secondary throttle shaft and, at its outer extremity, has a folded over portion 75, which journals a cross pin 76, and has a tail 77 to which is secured one end of a coiled tension spring 78. The other end of spring 78 is secured to a lug 79 on the end of secondary throttle actuating lever Z opposite link 65. Thus, the force of tension spring 78 is applied to lever 20 in the direction tending to close secondary throttle valve 18. Actuating lever 19 on the primary throttle is positioned, when the primary throttle valve is closed,rsubstantially on center relative to primary throttle shaft 55 and lug portion 79 of secondary lever 20. Thus, as primary throttle valve 17 is opened, bracket 75, 77 moves, harmonically, away from the dead center position so as to progressively relax spring 78 and, thereby, reduce the resistance applied by spring 78 to opening of secondary throttle 18.

A rod 85 is also pivotally connected to lug portion 79 of secondary throttle lever 20 and extends slidably through previously mentioned cross pin 76. A collar 86 is keyed to the free end of rod 85 and a coiled, cornpression spring 87 is compressed between this collar and a Washer 88 slidable on the rod for seating against cross pin 76. Spring 87 is substantially stiffer than tension spring 78, but is shorter so that its effect upon initial opening of the secondary throttle 18 is eliminated after a partial opening movement of primary throttle valve 17. Secondary throttle 18 cannot be opened as long as spring 87 is stressed. The purpose of spring 87 and overcenter rod 85 is to insure firm engagement of secondary throttle adjusting screw 90 with its stop 91, which fixes the closed position of secondary throttle 18, without the necessity of providing a strong return spring for this throttle. Thus, suction motor 60, in opening the secondary throttle, need exert only sufiicient force to counteract relatively light tension spring 78, the force of which is progressively reduced as the primary throttle valve is opened. The resistance of compression spring 87 likewise, will be progressively reduced, as the primary throttle valve is opened, until, when the primary throttle is fully opened, as in Fig. 3, its resistance to opening in the secondary throttle will be entirely eliminated. Thus, since relatively light forces need be exerted by the suction motor, this motor may be made relatively small and compact. Moreover, springs 78 and 87 do not materially increase the resistance to opening of the primary throttle.

The suction motor is shown mounted upon the carburetor by means of a bracket 95, secured to the cover 96 of the constant level chambers by means of bolts 97, and having a loop 98 gripping a cylindrical axial extension 99 on outer motor casing member 61.

It is believed that the operation will be readily apparent from the previous description. Fig.` l shows both the primary and secondary throttle valves in their closed or normal idling positions, as when the associated engine is not in operation or is idling. Fig. 3 shows a condition during the warm-up period with primary throttle valve 17 substantially fully opened, but with the secondary throttle valve 18 remaining closed due to the fact that the restricting position of choke valve 21 prevents the application of sufficient pressure difference to suction diaphragm 63 to overcome the force of tension spring 78, tending to hold the secondary throttle valve closed. This gure also shows stiff compression spring 87 completely separated from its supporting collar 86 on rod 85 so that its effect on the secondary throttle is completely eliminated.

In Figs. l and 2, with secondary throttle valve 18 fully closed, metering pin 38, controlling secondary metering orifice 33, will be lowered sufficiently to fully close this orifice and, therefore, prevent the flow of fuel through secondary main nozzle 35. This is important since the outlet of secondary nozzle 35 is located slightly below normal fuel level X-X in constant level chamber 26 so that a sufficient head is constantly applied to the metering orifice to cause spillage through main nozzle 35 except for cut-off valve 33. Operating linkage 39, 40, etc. of metering pin 38 is such that as secondary throttle 18 starts to open, metering pin 38, also, will start to open to insure prompt supply of fuel to the secondary mixture conduit. The location of secondary passage 35 and nozzle 35a wholly below the fuel level in secondary bowl 26, insures the prompt issuance of fuel from the secondary nozzle under a slight superatmospheric head, so that the flat spot which is frequently expe 'enced in current two-stage carburetors, due to the delay of fuel supply on the secondary side, is eliminated.

Fig. 4 shows the choke valve fully opened so as to eliminate its effect upon the suction motor, and primary throttle valve 17 opened approximately to the degree at which the speed of the associated engine and the velocity of air flow through venturi tube 16 produces a difference of pressure on the opposite sides of suction motor diaphragm 63 sufficient to start opening movement of secondary throttle 18. Tension spring 78 is considerably relaxed due to shortening in its movement from the position of Fig. 1, while compression spring 87 is fully expanded so that its resistance to initial opening of the secondary throttle valve is substantially eliminated.

Figs. 6 and 7 show a modification of the secondary throttle `operating mechanism in which means are provided fto limit the resistance to closing of the primary throttle which can be applied through the secondary throttle and its suction motor operating means. Such means comprises a spring capsule, generally indicated at consisting of an elongated cylinder 101 which slidably rcceives link 66a connected to suction motor stern 64. The link 66a has an enlarged head 102 at its free end between which *and the opposite end of a cylinder 101 there is compressed a coiled spring 103. Cylinder 101 has a detachable cap 104 at one end slidably receiving link 66o and, at its opposite end, is pinned to secondary throttle lever 20 at 105. A throttle return spring for 'the primary throttle valve is indicated schematically at 106.

Supplementary m'eans for limiting the effective opening pressure which may be applied to the secondary throttle, tending to resist its closing, consists of a by-pass tube 107 extending around diaphragm 63 of the suction motor and connecting m'otor chambers 69 and 70. A check valve 108 is received in valve casing 109 forming part of the by-pass and is normally urged against valve scat 110 by a coiled spring 111. Under normal conditions, the by-pass is closed by means of spring-pressed check 108. However, upon the existence of excessive depression in motor chamber 69 the valve can open to relieve this depression.

ln operation of this form of the invention, elements 66a, 101, 102, 103, and 104 form, in effect, a rcsiliently extensible link connecting the suction motor to secondary throttle lever 20. Thus, the resistance to closing of the secondary throttle when the primary throttle is substantially opened land the engine travelingv at a high rate of speed, is limited by the compressive strength of spring 103. Spring 103, of course, must be weaker than throttle return spring 106 which is the only instrumentality for applying closing force to the primary throttle valve and, through it, to the secondary throttle. Check valved bypass 197 assists the extensible link mechanism in accomplishing this result. Accordingly, prompt closing of the throttle valves is insured, when the laccelerator pedal is released by the operator, notwithstanding very high suction which may be applied to motor chamber 69 through suction passage, '7i at high engine speeds.

The means shown in the modification for limiting the opening force applied to the secondary throttle by means of the suction motor may be utilized with other types of two-stage throttle controls of the `general type utilizing a suction rnotor for opening the secondary throttle and Ithe extensible linkage land by-pass may be used individually if desired. Y

Primary and :secondary stages in either form may be provided in separately or independently mounted carburetors instead of the single carburetor, as shown. Moreover, each carburetion stage may be provided with a single, double, or any number of mixture conduits and corresponding charge forming and controlling elements.

The exclusive use of all modications as come within the scope of the appended claims is contemplated.

I claim:

1. A two-stage carburetion system comprising primary and secondary mixture conduits having primary and secondary throttle valves therein, a choke valve in the air entr-lance portion of said primary conduit and a Venturi tube in said primary conduit posterior to said choke valve, a manual control for said primary throttle valve, and means responsive to the difference of pressure between said air entrance portion of said primary conduit, posterior to said choke valve, and said Venturi tube for opening said secondary throttle valve as the rate of air flow through said Venturi tube increases.

2. A two-stage carburetion system for internal combustion engines comprising primary and secondary mixture conduits having primary and secondary throttle valves and mixing chambers, respectively, therein, `a choke valve in the air entrance portion of said primary conduit, a manual control for said primary throttle valve, means normally urging said secondary throttle valve closed, and suction motor means responsive to the difference of pressure between said air entrance portion of said primary mixture conduit, posterior to said choke vlalve, and said mixing chamber' for Iopening said secondary throttle valve as said primary throttle valve is opened and the velocity of air flow through said primary mixing chamber increases.

3. A two-stage carburetion system for internal combustion engines comprising primary and secondary induction conduits having primary and secondary throttle valves therein, Venturi tubes therein forming fuel and air mixing chambers, a choke valve in the air entrance portion of said primary conduit, means normally closing said secondary throttle valve when said primary throttle valve is closed, land suction motor means responsive to the difference of pressure in said primary conduit between the mixing chamber portion thereof and the part between' said p'ortion rand said choke valve for opening said secondary throttle valve as said primary throttle valve is lopened and the velocity of air flow through said primary conduit mixing chamber increases.

4. A two-stage carburetion system for internal cornbustion Iengines comprising primary and secondary induction conduits, each having air entrance and mixing chamber portions and fuel nozzles discharging, respectively, into said latter portions, primary and secondary throttle valves, respectively, in said conduits, a choke valve in the air entrance portion of said primary conduit, means for manuallyl controlling said primary Ithrottle valve, means for normally holding said secondary throttle valveclosed, yand a suction motor responsive to the pressure diierence between the air entrance land mixing chamber portions of said primary conduit for opening said secondary throttle vaive as said primary throttle valve opens and the air velocity through the primary mixing chamber `increases, the pressure connection between said suction motor and' said air entrance portion being posterior to said choke valve whereby opening of said 'secondary throttle is prevented when said choke valve is in a substantially restricting position; y

5. In a two-stage carburetion system for internal comb'ustio'r'r'V engines, primary and secondary induction con'- du'its yhaving primary and secondary throttle valves, respectively, therein,V manual means to control said primary throttle valve, said secondary throttle valve `automatically tending to open as said primary throttle valve opens and the rate of air flow through said primary conduit increases, and` means preventing opening of said secondary throttle valve until said primary throttle has opened a predetermined degree Vand the air flow through said primary conduit has attained a predetermined rate comprisingpirst and second pivoted levers rotatable, respectively, with said primary and secondary valves, and a spring stressed between said levers, said spring being applied to' a portion of said second lever so yas to tend to close said secondary throttle valve, and engaging a portion of said first lever substantially on center, when said primary throttle valve is closed and which moves in the direction tending to relax said spring as said primary throttle valve is opened.

6. A two-stage carburetion system for internal combustion engines comprising primary and secondary mixture conduits having, primary and secondary throttle valves', respectively, therein with tirst and second pivote-d levers movable, respectively, therewith, means including a motor for yieldably opening said secondary throttle valve responsive to increasing velocity of air flow through said primary conduit as said primary throttle valve opens, and means causing closing of said secondary throttle valve when said primary throttle valve is closed, comprising a tension spring member interconnecting .the ends of ysaid levers and forming a one-way operative connection between said levers and acting thereon in a direction tending to hold both valves closed.

7'. A two-stage carburetion system for internal cornbustion engines comprising primary and secondary induction conduits, primary and secondary fuel and air mixing chambers and throttle valves, respectively, in said conduits, a choke valve in the air inlet portion of said primary conduit, manual means for controlling said primary throttle valve, a suction motor operatively connected to said secondary throttle valve, said motor having operating pressure connections with said primary conduit, respectively, in the mixing chamber and air entrance portions thereof whereby said secondary throttle opens responsive to increase of air now through said primary conduit, actuating levers movable with said throttle valves, and spring means stressed between said levers for resisting opening of said secondary throttle variably as said primary throttle opens.

8. A carburetion system as described in claim 7 in which one of the suction motor pressure connections communicates with the air entrance portion of primary conduit between said primary mixing chamber and said choke valve whereby the pressure difference applied to said suction motor and accordingly the response of said secondary throttle to air ow changes is effected by the positioning of said choke valve.

9. A carburetion system as described in claim 7 in which one of the suction motor pressure connections communicates with the air entrance portion of primary conduit between said primary mixing chamber and said choke valve whereby the opening of said secondary throttle by said suction motor is prevented when said choke valve is in a substantially restricting position.

l0. A carburetion system 'as described in claim 7 further including a one-way operative connection between said valves for closing said secondary throttle valve when said primary valve is closed.

11. A two-stage carburetion system for internal combustion engines comprising primary and secondary induction conduits having fuel and lair mixing chambers and primary and secondary throttle valves respectively, therein, means responsive to suction in said primary conduit for opening said secondary throttle valve, said suction responsive means having operating passages exposing the same respectively to said primary mixing chamber and to a source of substantially atmospheric pressure under normal operating conditions, and choke valve means located in the air entrance portion of said primary conduit and operatively associated with one of said operating passages so as to prevent opening of said secondary throttle when said choke valve means is closed to start the associated engine.

12. A two-stage carburetion system for internal combustion engines comprising primary and secondary mixture conduits having primary and secondary throttle valves, respectively, therein with iirst and second pivoted levers movable, respectively, therewith, means for yieldably opening said secondary throttle valve responsive to increasing velocity of air ow through said primary conduit as said primary throttle valve openings, and means causing closing of said secondary throttle when said primary throttle is closed, comprising a spring member forming a one-way operative connection between said levers, said means for causing closing of said secondary throttle valve consisting of a spring member connected to a portion of said second lever so as to urge said secondary throttle valve closed when said element is stressed and connected to a portion of said iirst lever which `approaches an on-center position as said primary throttle is closed so as to increasingly stress said spring element.

i3. A two-stage carburetion system for internal combustion engines comprising primary and secondary-mixture conduits having primary and secondary throttle valves, respectively, therein with first and second pivoted levers movable, respectively, therewith, means for yieldably opening said secondary throttle valve responsive to increasing velocity of air ow through said primary conduit as said primary throttle valve opens, and means causing closing of said secondary throttle when said primary throttle is closed, comprising a spring member forming a one-way operative connection between said levers, said one-way spring member being inoperative to ciliect said secondary throttle valve in the vicinity of its closed position when said primary throttle is opened beyond a predetermined degree.

14. A two-stage carburetion system for internal cornbustion engines comprising primary and secondary mixture conduits having primary and secondary throttle valves, respectively, therein with lirst and second pivoted levers movable, respectively, therewith, means for yieldably opening said secondary throttle valve responsive to increasing velocity of air fiow through said primary conduit as said primary throttle valve opens, and means causing closing of said secondary throttle when said primary throttle is closed, comprising a spring member forming a one-way operative connectionV between said levers and additional spring means resisting opening of said secondary throttle valve, both said spring member and said additional spring means applying decreasing closing force to said secondary throttle valve as said primary throttle valve is opened.

15. A two-stage carburetion system for internal cornbustion engines comprising primary and secondary mixture conduits having primary and secondary throttle valves, respectively, therein with first and second pivoted levers movable, respectively, therewith, means for yieldably opening said secondary throttle valve responsive to increasing velocity of air ow through said primary conduit as said primary throttle valve opens, and means causing closing of said secondary throttle when said primary throttle is closed, comprising a spring member forming a one-way operative connection between said levers and additional spring means resisting opening of said secondary throttle valve, said spring member becoming operative to urge said secondary throttle closed as said primary throttle approaches closed position, and said additional spring means being connected to a position of said first lever which moves away from dead center position as said primary throttle is opened, so as to progressively decrease the resistance applied to said secondary throttle valve thereby.

16. A two-stage carburetion system for internal combustion engines, comprising primary and secondary mixture conduits having primary and secondary throttle valves, respectively, therein with rst and second pivoted levers movable, respectively, therewith, means for yieldably opening said secondary throttle valve responsive to increasing velocity of airflow through said primary conduit as said primary throttle valve opens, and means causing closing of said secondary throttle when said primary throttle is closed, comprising a tension spring interconnecting said levers and whose axis varies so as to tend to decrease the force acting to open said primary throttle ari/ increase the force acting to close said secondary throttle during closing of said primary throttle. 17. In a carburetor, a secondary mixture conduit having a throttle valve therein, stop means limiting the closing movement of said valve, means to open said valve, and manual means for closing said valve comprising a iirst pivoted lever movable with said valve and a manually actuated pivoted lever means including a second lever, a link connected with said first pivoted lever, a lost motion pivoted connection between said link and second lever operative to close said valve as said link and second lever approach a dead center position relative to said lost motion pivoted connection, and a tension spring interconnecting the ends of said tirst and second levers and effective in the dead center position of said levers to yieldably maintain said valve closed.

18. A two-stage carburetion system for an internal combustion engine comprising primary and secondary induction conduits with primary and secondary mixing chambers and throttle valves, respectively, therein, manual means to open said primary throttle valve, a throttle return spring for closing the primary throttle valve, a suction motor having pressure and suction chambers and responsive to suction in said primary mixing chamber for opening said secondary throttle when said primary throttle valve is opened, interconnection means between said throttle valves for causing closing of said secondary throttle valve as said primary throttle valve is closed, a by-pass between the pressure and suction chambers of said motor, and a resiliently seated check valve in said by-pass opening toward said suction chamber to limit the effective force applied by said motor.

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